1. Field of the Invention:
This invention relates to zoom lens mountings and more particularly to zoom lens mountings with a stop having a prescribed inner diameter to shut off harmful light and arranged in a lens system to move axially in a relation that is differential to the lens components axially movable for zooming in a part of the zooming range.
2. Description of the Prior Art
From the past, the zoom lens design has had a difficult problem of removing the comatic aberration resulting from the excess of the marginal light amount. Particularly in recent years the concave and convex or two component type has found its use in wide angle zoom lenses. In this type zoom lens, because its front lens component is strongly diverging, the off-axis light bundle arriving at the middle zone of the image format becomes so excessive that it is very difficult to remove the comatic aberration, and the slope of lowering the illumination at the image plane becomes very steep, intensifying the drop of the corner illumination.
As the means for eliminating the above-described drawbacks, there is presently known a harmful light shutting stop arranged in three different ways: (I) It moves as a unit with either one of the front and rear lens components; (II) It moves in differential relation to any of the front and rear lens components; and (III) It is positioned in a space within a lens component which remains stationary during zooming. Among these, the second way (II) has the greatest harmful light shutting effect and is disclosed, for example, in Japanese Laid-Open Patent Application No. SHO 54-23555. But in addition to the cam means for controlling the movement of the front and rear lens components there is need to employ another separate cam means for controlling the movement of the stop. The space the second cam means occupies has been a barrier to promoting minimization of the bulk and size of, and a reduction of the weight of, the zoom lens.
Also, the third way (III) is disclosed in Japanese Laid-Open Patent Application No. SHO 56-67814 where the stop is positioned in a space between the front and rear lens components and remains stationary relative to the lens mount surface, whereby the use of the cam means for the stop is no longer necessary. But the stop position is limited to a point just in front of the rear lens component when in the telephoto end. When zoomed to the wide angle end, the stop takes its position as deviated from the optimum point toward the front lens component. In order to admit a sufficient light bundle to arrive at the margin of the image format when in the wide angle end, the diameter of the stop must be increased. Thus, to obtain a sufficient effect becomes difficult.
Further, as the light shielding device of the zoom lens mounting the technique of Japanese Utility Model Publication No. SHO 56-22167 is known.
Said device has a light shielding member arranged ahead of the zoom lens optical system to move axially during zooming, wherein the cam for controlling the movement of the light shielding member is provided on the common sleeve of the cams for controlling the movements of the zoom lens components but in separation from the latter.